1. Field of the Invention
The present invention relates to methods and apparatus for measuring the voltage potential of a charged photoreceptor, and in particular to methods and apparatus for measuring the voltage potential of a charged photoreceptor in a xerographic imaging device capable of printing images having two or more colors.
2. Description of Related Art
A typical electrophotographic printing machine (such as a photocopier, laser printer, facsimile machine or the like) employs an imaging member (e.g., a photoreceptor) that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material (liquid or powder)into contact with the latent image. The developed image (toner image) recorded on the imaging member is transferred to a support material such as paper either directly or via an intermediate transport member. The developed image on the support material is generally subjected to heat and/or pressure to permanently fuse the image to the support material.
Multicolor printing machines include printing machines which can print with highlight color (usually black and one other color such as, for example, red, green or blue) and machines which can print with process color (usually four different colors, such as black, yellow, magenta and cyan).
There is a class of color printer which builds up multicolor images of toner on the photoreceptor and then transfers this multicolor toner image in one step, as opposed to multiple transfer steps of the individual colors separately. Within this transfer class of color machines there are two types of multicolor electrophotographic printing machines which are typically employed to form highlight or process color images. One type of single transfer multicolor electrophotographic printing machine is known as a multi-pass color printer. The multi-pass color printer typically has an imaging member (such as, for example, a photoreceptive drum or belt) having a single charger (for charging the imaging member to a uniform voltage potential), exposure device (for forming a latent image on the charged imaging member)and developer device (for developing the latent image into a toner image). In order to form multicolor images, the imaging member must rotate multiple times (i.e., one time for each color in the image). The second type of single transfer multicolor printer is known as a single pass multicolor printer. The single pass printer includes a plurality of charging devices, exposing devices, and developing devices located around the periphery of the photoreceptor, and corresponding in number to the total number of colors to be formed in the image. For example, a single pass printer capable of highlight color printing could include two sets of charging devices, exposing devices and developer devices, while a single pass printer capable of printing images with four colors would include four sets of charging devices, imaging devices and developer devices.
U.S. Pat. Nos. 4,833,503 to Snelling (Xerox Corporation) and 4,791,452 to Kasai et al. (Toshiba) illustrate single pass multicolor electrostatic printing machines which include a plurality of charging, exposing, and developing devices corresponding in number to the total number of colors in the final image. Accordingly, U.S. Pat. Nos. 4,833,503 and 4,791,452 are incorporated herein by reference in their entireties.
Typically, multicolor electrophotographic printing machines form the single color component images of a multicolor image on top of each other. In other words, the latent image for the second component color is formed directly over the toner image of the first component color, etc. During each exposure operation, the portion of the photoreceptor which is not exposed to light (typically referred to as background areas), are not discharged, and ideally should remain at the voltage potential to which the photoreceptor was charged by the previous charging device. However, all photoreceptors are somewhat conductive, and therefore experience a decrease in voltage potential over time even when they are not exposed to light. This decrease in voltage potential is known as dark decay. It is important to know the dark decay characteristics of a photoreceptor, especially in multicolor printing machines, for example, so that the photoreceptor can be recharged to a proper voltage potential (for forming second, third, fourth, etc. component images of a multicolor image) so that the exposure devices can be set at the proper light intensity, and so that the bias voltage of the developer devices can be set at a proper level. Additionally, the dark decay characteristics of the photoreceptor should be monitored over time because they change as the photoreceptor ages and is used a large number of times.
In order to determine the voltage potential of a photoreceptor at points of interest (e.g., the point of recharging, the point of light exposure, the point of development, etc.), some printing machines include devices for directly measuring the voltage potential of the photoreceptor at these points. For example, U.S. Pat. No. 4,998,139 to May et al. (Xerox Corporation), the disclosure of which is incorporated herein by reference, uses an electrostatic voltmeter (ESV) between the exposing device and the developer devices in a tri-level printing machine. The photoreceptor potential measured by the ESV is used to control the ROS device which exposes the photoreceptor to a light image. Additionally, the abovementioned U.S. Pat. No. 4,791,452 includes sensors after each of its two charging devices in order to determine the voltage potential of the photoreceptor at these points.
However, conventional devices for measuring the voltage potential of a photoreceptor, such as the above-described electrostatic voltmeters, are expensive. Accordingly, many commercial devices do not include ESVs. For example, the Panasonic FPC-1and the Konica 8028/9028 do not include ESVs. In such products, a service technician sets up the printer by hand, with the hope that there will be no variations, drifts, etc. over time and between copies. The penalty for such measures can be poor print quality stability and maintenance.
The quality of color images produced by printers which do not include ESVs, however, degrades over time as the dark decay characteristics of the photoreceptor changes.